In recent years, synthetic resinous materials such as those mainly consisting of nylon, ABS resin and so forth are becoming popular as the materials for the golf club heads, particularly the heads of clubs called "wood", "driver" and "spoon", besides the natural wood material such as persimmon, cherry and so forth. Usually, the golf club head of a synthetic resin is formed integrally by means of a pair of split mold elements. Therefore, certain problems are encountered in the reduction of weight and weight balancing of the head body, if the size and weight of the club head of a plastic are selected to be equal to those of wooden club heads, and if a sole plate is used to prevent the damage of the sole plate and to obtain the balance of weight so as to attain performance and characteristics equivalent to those of the wooden club heads. The prior art deals with the formation of an internal hollow for the golf club head, and includes U.S. Pat. No. 3,556,532 to James E. Ballmer, dated Jan. 19, 1971, entitled PLASTIC GOLF CLUB HEAD WITH CAVITIES THEREIN TO SOUND LIKE A WOODEN CLUB HEAD, and U.S. Pat. No. 3,140,094 to Donald P. Hings, dated July 7, 1964, entitled EPOXY RESIN GOLF CLUB HEAD INTEGRALLY CURED WITH A SHAFT WRAPPING OF GLASS FIBER MATERIAL.
It has been proposed to adopt a hollow structure of the golf club head in order to reduce the weight of the same. This, however, imposes the following problems. Namely, for attaching a sole plate as in the case of the wooden club heads, the position of the hollow tends to be offset with respect to the head body because of the necessity of the mounting space for mounting the sole plate, resulting in an unbalance of weight of the head body. The weight unbalance of the head body inconveniently produces a moment of inertia at the time of impact to make it difficult to control the direction of flight of the ball. Particularly, in the golf club mounting a sole plate, the hollow can be formed only in one portion of the club head under the sole plate so that the weight center of the club head is shifted undesirably towards the toe portion to produce a moment of inertia in the head body. In addition, the "sweet spot" is restricted undesirably to make the golf club difficult to use.